Wafer probe for measuring plasma and surface characteristics in plasma processing enviroments

ABSTRACT

There is provided by this invention a wafer probe for measuring plasma and surface characteristics in plasma processing environment that utilizes integrated sensors on a wafer substrate. A microprocessor mounted on the substrate receives input signals from the integrated sensors to process, store, and transmit the data. A wireless communication transceiver receives the data from the microprocessor and transmits information outside of the plasma processing system to a computer that collects the data during plasma processing. The integrated sensors may be dual floating Langmuir probes, temperature measuring devices, resonant beam gas sensors, or hall magnetic sensors. There is also provided a self-contained power source that utilizes the plasma for power that is comprised of a topographically dependent charging device or a charging structure that utilizes stacked capacitors.

This application is a divisional application of, and claims the benefitand priority of, U.S. patent application Ser. No. 10/194,526 filed Jul.12, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to apparatus and methods in whichsurface-based sensors measure incident charged-particle currents,charging voltages, temperatures and other physical parameters at a workpiece surface during plasma processing, and more particularly to asemiconductor wafer utilizing surface-based sensors to provide real timemeasurement of plasma characteristics adjacent to the wafer surface aswell as select physical properties during plasma processing.

2. Brief Description of the Prior Art

Spatial and temporal variation in plasma characteristics and the workpiece surface temperature can strongly influence the performance andyield of plasma-based processes, such as those encountered insemiconductor manufacture. In such processes, variations in physicalplasma parameters that occur adjacent to the process work piece directlyimpact process metrics which may include the following: (1) etch ratesand etch profile control, (2) surface charging effects and device orfilm damage, and (3) thin film deposition rates, density, coverage,morphology, stress and adhesion. Some common plasma parameters thatdrive surface processes on a work piece, such as a semiconductorsubstrate wafer, include charged-particle density and flux (ion andelectron density), apparent electron temperature, ion energies, neutralgas temperature, density and flux of reactive gas species, and plasmaradiative emissions. It is also known that surface temperature of thework piece or wafer can play a very critical role in many of the surfacereactions and results of the plasma process.

Because of the criticality of both plasma characteristics and substratetemperature and their impact on process yield, several workers haveattempted to monitor plasma characteristics and surface temperaturesduring processes by means of diagnostic probes that are directly mountedto a work piece, such as a semiconductor wafer substrate. In thesedevices, diagnostic probes such as thermocouples, DC-biased electricalprobes, ion energy analyzers, and surface charging collectors have beenused to measure spatial and temporal variation of surface temperature,selected plasma parameters, and plasma-induced charging effects. Onesuch device is the Stanford Plasma On-wafer Real Time (SPORT) probe asdescribed in an article by S. Ma and J. P. McVittie in the proceedingsof the 1996 International Symposium on Plasma Process-Induced Damage pg.20-23. The SPORT probe is capable of measuring electrostatic chargingand plasma-induced currents at the wafer surface. The SPORT probeutilizes large conductive pads placed on a thick oxide layer of asilicon wafer. Polysilicon leads make direct current contact to the padsand the silicon substrate. Wire leads connected to the edge of the wafercarry current and voltage signals outside the plasma-processing chamberto a low pass RF filter to a dc measurement circuit. By means of theexternal measurement circuit, plasma induced charging voltages aremeasured between the pads and the substrate in order to quantify plasmainduced electrostatic charging effects that could result in damage toelectrically sensitive semiconductor device structures during plasmaprocessing and fabrication.

Another apparatus is described in U.S. Pat. No. 5,801,386 issued toValentin N. Todorov et al. This patent discloses an apparatus thatcomprises a plurality of conductive collector pads for detection ofplasma induced ion currents and self-biased voltages. The collector padsare arranged in an array so that plasma-induced properties of ioncurrent and self-bias voltage can be spatially resolved over the wafersurface in real time. Each collector pad is connected to a conductivelead that extends outside the chamber to an external data acquisitionsystem.

Also in U.S. Pat. No. 5,959,309 entitled “Sensor to Monitor PlasmaInduced Charging Damage”, Tsui, et al. describe a discrete monitoringcircuit that measures the plasma-induced voltage and currents to asampling pad or antenna that is in communication with a ground orcommon. In this device, the sampling pad is connected to ground througha blocking diode, a blocking transistor, and a storage capacitor. Oncethe monitor is exposed to the plasma, the voltage between the chargedpad or antenna and the electrical common or ground is recorded bycharging a storage capacitor. The workers also disclosed how a pluralityof these monitors, each with different loading resistances, can beintegrated onto a single chip to measure the magnitude of the chargingvoltage and the plasma-induced current between the antenna and common orground of the chip. The charging voltage and pad-to-common currents aredetermined by electrically measuring the voltages of the storagecapacitors after the sensor or chip is removed from the plasmaprocessing environment.

Freed et al. describe the development of sensor methods in “AutonomousOn-Wafer Sensors for Process Modeling, Diagnostic and Control” (IEEETransactions on Semiconductor Manufacturing, Vol. 14, No. 3, pp225-264). This paper describes the basic design challenges faced in thedevelopment of an in situ or in-line wafer sensor including power sourceconcepts, wireless communications methods, and electrical isolation ofon-wafer electronics. In their examples, they illustrate two designconcepts. In the first design concept is an on-wafer thermistor sensorpowered with re-chargeable batteries and voltage regulator. The designalso includes an A/D converter and LED optical communication electronicsfor transferring data off the wafer in a thermally elevated processenvironment and a plasma etching environment. In another version of thedesign, the workers illustrate how a van der Pauw sheet resistancedevice may be adapted with CMOS processing methods for measuringpolysilicon etch rates. They demonstrate the viability of this sensorwith a wired wafer as applied to a XeF₂ (non-plasma) etching reactor.These devices have varying degrees of effectiveness in monitoring thewafer temperature or the characteristics of a plasma body adjacent tothe wafer when disposed in a plasma processing environments. However,all the examples of the prior art have several limitations that restricttheir use for obtaining real-time plasma and substrate temperaturemeasurement within a plasma processing system. Many of these measuringdevices are intrusive in that they require the use of wires into theplasma processing system and others are passive recording devices thatcannot make real-time measurements. Also, those devices that do not useexternal wires are limited in on-time operation and power supply currentdraw since they rely entirely upon on-board battery power sources thathave limited milliamp-per-hour capacity or limited sustainable tricklecurrent capacity when attempting to power a sizable array of sensors,microprocessor(s) and wireless communication subsystems. Moreover, inthe context of these in situ measurement apparatuses, none of the priorart teachings discuss in detail how to devise a sensor capable ofobtaining plasma measurements, such as charged-particle (ion orelectron) fluxes, densities and energies that can be adapted to awireless sensing apparatus.

It would be desirable if there were provided a surface-based sensorapparatus that could make spatially resolved, real-time measurements ofplasma properties adjacent to the surface of the apparatus, as well asother properties such as surface temperature. It would also be desirableif the device were non-invasive to the plasma process and if thetime-dynamic data recorded by the device could be either transmitted inreal-time through a wireless interface or, alternatively, be recordedfor downloading once the sensor apparatus is removed from the plasmaprocess chamber. It would be further desirable if the device had aself-contained power supply means that did not rely entirely upon thelimited lifetime or trickle current ratings of a battery or alternativeconventionally power source.

SUMMARY OF THE INVENTION

There is provided by this invention an apparatus for making real timemeasurements of incident plasma currents, charging surface voltages, andother plasma related parameters as well as surface temperatures within aplasma processing environment. The apparatus is generally comprised ofat least one integrated sensor circuit mounted on a work piece such as asilicon wafer substrate. The sensor is comprised of either a dualfloating probe to measure ion currents from the plasma, a topographicaldependent charging structure to measure plasma induced surface chargingeffects, filtered photodiodes to measure optical emissions signals, athermal sensing device to monitor surface temperature or a combinationthereof. The sensor inputs are transmitted to a central microprocessorand transceiver that is provided for processing sensor signals, memorystorage, and real-time transmission of data via infrared- or rf-wirelesscommunication to a receiver outside the plasma chamber. To power theapparatus, a battery is contained within the apparatus to provide powerto the integrated sensor devices, microprocessor and wirelesstransceiver. Alternately, the apparatus may include one or moretopographically dependent charging structures to electrostaticallycouple power from the plasma boundary that is then regulated and used toprovide all or part of the power to the apparatus electronics. Theapparatus is particularly useful in spatial and real-time monitoring ofplasma and substrate conditions in plasma-based non-depositing processessuch as etching, photo-resist stripping or surface cleaning, but couldbe applied to some plasma-based deposition processes with theappropriate configuration or adaptation of the integrated sensingdevices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a simplified cross-sectional view of a semiconductormanufacturing process chamber in which to utilize the sensor apparatusfor measurements across the surface of a wafer or work piece;

FIG. 2 a illustrates the sensor apparatus of the invention asmanufactured on a silicon wafer substrate and which is comprised of aplurality of integrated circuit sensors and a central microprocessorwith wireless communication capability;

FIG. 2 b illustrates the sensor apparatus of the invention asmanufactured on a vacuum compatible work piece and which is comprised ofa plurality of integrated circuit sensors and a central microprocessorwith wireless communication capability;

FIG. 3 illustrates a schematic diagram that is representative of theelectrical architecture of invention in which output of a discretesensor is collected by microprocessor with signal processing andwireless communication capability;

FIG. 4 illustrates a schematic diagram that is representative ofelectrical architecture of invention in which the analog output of aplurality of discrete sensors are transmitted to a centralmicroprocessor with multiplexing, analog-to-digital conversion, signalprocessing and wireless communication capability;

FIG. 5 illustrates a schematic diagram that is representative ofelectrical architecture of invention in which a plurality of discretesensors have an integrated analog-to-digital and signal conditioningcapability and are digitally tied to a central microprocessor withsignal processing and wireless communication capability;

FIG. 6 a illustrates the diagram of an integrated sensor which includesa thermocouple or thermistor device for monitoring surface temperatures,a dual-floating Langmuir probe for monitoring ion currents and apparentelectron temperature, one or more topographical dependent chargingstructures for monitoring plasma-induced surface charging effect;

FIG. 6 b illustrates the diagram of an integrated sensor which includesa thermocouple or thermistor device for monitoring surface temperatures,a dual-floating Langmuir probe for monitoring ion currents and apparentelectron temperature, one or more topographical dependent chargingstructures for monitoring plasma-induced surface charging effect, signalconditioning circuitry and a microprocessor for analog-to-digitalconversion of sensor output and serial communication;

FIG. 7 illustrates a block diagram of a dual floating probe (DFP)structure with a typical current-voltage response curve when exposed toa plasma and voltage bias signal;

FIG. 8 illustrates a capacitively-coupled circuit for pulsing the DFPdevice and the typical current response when exposed to a plasma;

FIG. 9 illustrates a topography dependent charging (TDC) structure inthe presence of a plasma and means by which a voltage is induced on thestructure by a plasma;

FIG. 10 illustrates how multiple TDC structures can be ganged togetherto provide power to one or more DC-to-DC converter sub-components inorder to power multiple integrated sensors;

FIG. 11 illustrates an a surface charging structure utilizing stackedcapacitors;

FIG. 12 a illustrates how a TDC structure sensor can be dynamicallyloaded to obtain current and voltage characteristics for the purpose ofmeasuring current flux and surface charging effects resulting from iontransport within the TDC structure; and

FIG. 12 b is a graph that illustrates a load line and power lineobtained from the loaded TDC device diagnostic circuit.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 there is illustrated an apparatus10 that is capableof making real-time measurements of incident plasma current flux andsurface temperatures of a work piece in a plasma processing system 12.In this particular illustration a diagnostic probe 14 is comprised of asilicon wafer substrate that incorporates plasma probe and surfacetemperature diagnostic circuitry and wireless communications and astored power system. In the preferred embodiment, the probe 14 ispowered up outside the plasma processing system 12 to enable diagnosticcommunications prior to use for real-time measurements. The plasmaprocessing system of FIG. 1 is one of many possible plasma processingsystems and is presented here to illustrate the function and use of thepresent invention. The processing system is comprised of a vacuumprocessing chamber 16, a pumping manifold 18, a plasma source electrodemounted to the top of the chamber 20, a gas delivery manifold or gasshroud 22, gas flow and power connections to the plasma source electrode24, a wafer chuck 26 with clamp ring 28, RF power, fluid coolant,lifting pin, and helium backside-cooling services to the chuck 30, and aload lock transfer stage with mechanical robotics 32 to manipulate thewafer into the chamber 16 onto the chuck 26. Plasma 34 is ignited toperform an etching or deposition process on the surface of the wafer atwhich time the apparatus sensors and microprocessor is triggered tocollect surface properties or plasma properties in close proximity tothe apparatus surface in real time. An on-board wireless transceiversystem 36 is used to communicate data and instruction with a basestation transceiver 38 outside the plasma processing system. The basestation transceiver 38 allows for communication of data and instructionsbetween the software of the external computer 40 and the probe 14 inreal time. Alternatively, it is possible to have the probe collectinformation inside the process and then download data once it is removedfrom the process chamber.

In the preferred embodiment the sensors can be fabricated on asemiconductor wafer such as a 200 mm or 300 mm diameter silicon wafer.However, they may also be fabricated on any process work piece such as aceramic, plastic, metal or glass work piece surface that can beintroduced into the vacuum chamber. These configurations are illustratedin FIGS. 2 a and 2 b respectively. FIG. 2 a shows how an array ofspatially distributed sensor elements 42 are disposed on thesemiconductor wafer 14 with multiple interconnects 44 for communicationand/or power distribution from a central processor and wirelesscommunication subsystem 46. FIG. 2 b shows how a similar device withsensors 42, interconnects 44 and central processor with wirelesscommunication module 46 could be patterned on any flat article or workpiece 14 to be placed into the plasma processing system.

There are three specific architectures of interest that may be used inwithin the apparatus of the present invention. These are given as

-   -   (1) single-point sensing devices with dedicated single        conditioning circuitry, analog-to-digital (A/D) converter,        micro-processor, wireless communications and power source;    -   (2) multiple-point or distributed sensing devices with analog        and control signals multiplexed to a central signal condition        circuitry, A/D converter, micro-processor, wireless        communications and power source; and    -   (3) multiple-point or distributed sensing devices with dedicated        signal conditioning circuitry, A/D converter, local power source        and with serial communications to a central micro-processor,        wireless communications and power source.

The choice of any particular architecture is dependent upon the need forcollecting real-time spatial information, use of materials andmanufacture constraints, and data collection issues related to noiseimmunity and speed of data collection over a distributed serial ormultiplexed electrical system. These basic architectures are describedin more detail hereinafter.

There are three sensor devices or means that are of particular interestfor measurement of parameters at the work piece surface or from theadjacent plasma body. These sensors are used in examples to illustratethe operation and function of the apparatus. These sensors are:

-   -   a) a thermal sensing element, such as a thermocouple or        thermistor, for measuring work piece temperatures;    -   b) a dual-floating Langmuir probe (DFP) for measuring plasma ion        current fluxes, apparent electron temperatures and estimates of        ion and electron densities;    -   c) a topographically dependent charging (TDC) devices for        measuring surface charging effects and for coupling power from        the plasma sheath.

The common aspect of these particular sensor devices is that they areexamples of sensing devices that can be electrically floated and thusthey are viable for sensing process properties on a work piece when thework piece is electrically floating or if there is an active RFself-bias applied to the work piece. In such cases there can be noground connection between the processing system ground and the workpiece. These sensing devices may be used alone or in certaincombinations depending upon the needs or the application. While thesethree specific devices are used to describe the apparatus or the presentinvention, there are other possible sensors that may be used in thecontext of the apparatus, which are described in the followingspecification.

FIG. 3 is an illustration of a single point sensing architecture of theapparatus of the present invention. The sensors are mounted to a surfaceof the wafer 14 such that they are exposed to the flux ofcharged-particle species from the plasma 34. In this case the multiplesensors include electrodes to a dual-floating Langmuir probe 48, asurface temperature sensor 50, topographically dependent chargingsensors 52 with varying aspect ratio and a TDC bank 54 that feeds avoltage to a DC-DC converter and regulator 56 for optional or auxiliarysensor power. A protective package 58 is used to shield most all otherelectronics from the exposure to the plasma environment. The supportingelectronic architecture includes a sensor signal power and conditioningcircuit 60, an A/D converter section for analog signals 62, a centralmicroprocessor 64, additional memory 66, a crystal oscillator 68 and thewireless transceiver 36 that includes a radio antenna 70. Alternately,the transceiver maybe infrared LEDs 72 as a means for wirelesscommunications. The wafer also includes an energy source 74 which may bea capacitor or battery that powers the system through a magnetic reedswitch 76 and DC-DC regulator 56. This particular arrangement ispreferred when temporal data from the sensors is a critical single pointor zone that is required to monitor the phenomenon in real-time.

FIG. 4 is an illustration of a multiple-point or distributed sensingarchitecture wherein several sensor signals are multiplexed to a centralsignal condition circuitry, A/D converter, microprocessor. As with theprior architecture, the sensors are mounted to a surface of the wafer 14such that they are exposed to the flux of charged-particle species fromthe plasma 34. However in this arrangement, several multi-sensor modules78 are separate from the electronics enclosure 58, yet havecommunication lines 80 to a multiplexing component 82. In this manner,the signal from several multi-sensor modules, which may be built up fromrepeated hybrid devices or system-on-chip modules, can be spatiallypolled through an array of distributed communication lines 84. Eachmulti-sensor module 78 includes electrodes for the DFP 48, the surfacetemperature sensor 50, and a TDC sensor 52 for recording surfacecharging. An optional TDC bank 54 may be included in a separate modulepackage 88 in order to power the DC-DC converter and regulator 56 foroptional or auxiliary sensor power. In this multiplexing arrangement,the supporting electronic architecture includes the multiplexer 82, thesensor signal power and conditioning circuit 60, the A/D convertersection for analog signals 62, the central microprocessor 64, additionalmemory 66, the crystal oscillator 68 and wireless transceiver 36 thathas either the radio antenna 70 or the infrared LEDs 72 as a means forwireless communications. As with the first architecture, the secondarchitecture includes an energy source 74 which may be a capacitor orbattery that powers the system through the magnetic reed switch 76 andDC-DC regulator 56. This particular arrangement is preferred whentemporal and spatial data from several replicated sensor modules 78 canbe collected with little risk of noise and interference along patternedcommunication lines 80 and 84. It would be economical to use thisparticular architecture as the collective elements of the centralmicroprocessor, signal conditioning circuit, and multiplexer could beincorporated into a single Application Specific Integrated Circuit(ASIC) component or, alternatively, into an integrated multi-chip hybridcircuit. Also, with this architecture, it is possible to use ahermetically sealed electronics package for enclosure 58 andelectrically insulated connections to deposited or printed conductivetraces for communication lines 80 and 84 and connections 94 between TDCauxiliary power bank 54 and DC-DC regulator 56. Such preferred methodsof fabrication would allow the apparatus to be fabricated on asemiconductor wafer 14 with materials that are compatible withsemiconductor plasma processing environments.

FIG. 5 is an illustration of a multiple-point sensing architecturewherein several replicated sensing modules with dedicated signalconditioning circuitry, A/D converter, and optional TDC-base powersource are connected to a central microprocessor and wirelesstransceiver through shared serial communications. In this version of thearchitecture several sensor modules or subsystems 78 are mounted to asurface of the wafer 14 such that the sensing components are exposed tothe flux of charged-particle species from the plasma 34. Each sensormodule 78 is comprised of a DFP sensor 48, a temperature sensor 50 andat least one TDC sensor 52. An additional bank of TDC devices 54 may beused to couple power from the plasma boundary to a DC-DC regulator 56 tolocally power the sensor module when operated in the presence of aplasma. In order to make local measurements, each sensor module 78 has asensor signal power and conditioning circuit module 96, a localmicroprocessor 98 with A/D conversion 100 and isolation for serialcommunications 102. Data is transmitted to the central processor 64within package 58 through communication lines 103 and serial interface106 as distributed along a shared serial bus 108. As with previousapparatus architectures, the main microprocessor module 64 hasadditional memory 66, a crystal oscillator 68 and a wireless transceiver36 that has either a radio antenna 70 or infrared LEDs 72 as a means forwireless communications. The module also includes an energy source 74which may be a capacitor or battery that powers the system through amagnetic reed switch 76 and DC-DC converter 56. This arrangement ispreferred when temporal and spatial data from several replicated sensormodules 78 can be fabricated from an ASIC or hybrid circuit component.The digital communications between the sensors and the central processorshould provide good immunity to noise and address common mode voltageissues since analog signals are not being spatially routed over theapparatus surface. Moreover, the localized sampling and storagecapability of the local microprocessors 98 may enable faster samplingrates and real-time transmission of the sensor data when compared to amultiplexed architecture as illustrated in FIG. 4.

As mentioned earlier, the surface sensor devices could be fabricatedfrom a hybrid circuit components or an ASIC to form the replicatedsensor modules 78 as shown in FIGS. 4 and 5 . Such modularsub-components could be mounted on a discrete circuit substrate such asa ceramic or high temperature thermal plastic with output leads bondedto patterned interconnections on the wafer 14. Alternatively, the sensormodules and interconnections could be patterned directly into thesurface of a silicon wafer 14 to provide an integrated sensor array.FIGS. 6 a and 6 b show a representative top view of sensor modules. Inparticular FIG. 6 a shows a layout of a sensor module 78 as applicableto the multiplexed architecture of FIG. 4. This sensor module includesDFP collection probes 48, surface temperature sensor 50 and fourseparate TDC sensors 52 along with bondable pad connections 110 to thevarious sensors. FIG. 6 b shows a layout of a sensor module 78 asapplicable to the serial architecture of FIG. 5. Along with the DFPcollection probes 48, surface temperature sensor 50 and TDC sensors 52,this module also includes a secondary TDC bank 54 for local powergeneration, DC-DC converters 56, signal conditioning circuitry 60, alocal microprocessor 98 with A/D 100 and isolated serial communications102, and bondable pads connections for serial communications 112 anddistribution of auxiliary power 114.

With respect to surface temperature sensing devices, there are threetypes that may be used. These include 1) thermocouples and thermistorsthat are encapsulated in integrated circuit package for surface mountingto the wafer and which provide a varying bi-metal voltage or electricalimpedance with respect to temperature, 2) IC packaged thermocouple orthermistors which have integrated electronics and which provide adigital output of temperature and 3) thermocouple or thermistor devicesthat have been patterned and integrated into the surface of the waferwith conventional semiconductor-IC manufacturing methods. Those thermalsensors that are based on IC packages are economically advantageous inarchitectures where hybrid electronics are used, but because of theirlocal thermal mass, they provide only an estimated measure of the wafersurface temperature. Sensors that are fabricated directly into the wafersurface are preferable because of their exactness, but their use caninvolve considerable cost in pattering and manufacture when compared toIC surface mount devices.

Detailed knowledge of plasma parameters in proximity to the wafersurface can be extremely helpful in understanding the dynamics ofplasma-based processes. Spatial and temporal measurements of plasmaparameters such as ion current flux, charged-particle density, electronenergies (or apparent electron temperatures) and ion energies arephysical parameters of the plasma that directly influence rates andquality of surface modification and material deposition and etching. Inthe preferred embodiment of this invention, the double-floating Langmuirprobe or DFP is used to collect some of these plasma parameters.Described in the early 1950s, the double-floating Langmuir probe allowsone to obtain I-V characteristics of a plasma between two conductiveprobes that are allowed to electrically float apart from an electricalcommon or ground. A detailed description of the DFP diagnostic techniqueand theory has been given by Swift, J. D. and Schwar, M. J. R., ElectricProbes for Plasma Diagnostics, Chapter 7, pp. 137-155, (Elsevier, N.Y.,1969).

FIGS. 7 a and 7 b illustrate the basic working principles of the DFP.Two planar conductive probes 48 are isolated from the wafer surface 14and exposed to plasma 34. A floating bias potential 116 is inducedbetween the probes to force a net current 118 through the plasma and DFPcircuit. When placed at relatively high bias potentials, the net DFPcurrent is limited by ion current flux 120 to the negatively biasedprobe surface due to the rejection of electrons currents 122 to theprobe; this is known as the ion saturation current. The ion current 124and electron current 126 to the second probe compensate in order to sumall plasma currents to zero. When the probes are nearly symmetric andthe plasma is relatively uniform across the surface, an I-V trace 128 isproduced that is similar to back-to-back diode I-V characteristics shownin FIG. 7 b. The saturating current regions 130 and 132 correspond toion saturation currents 120 and 124 respectively. The intermediateregion of the I-V curve 134 is determined by the electron currents 122and 126 between the probes and may be used to determine the apparentelectron temperature, <T_(e)>, which is indicative of “high-energy”electron population of the electron energy distribution as manifested atthe boundary of the wafer surface 14.

The current characteristics versus bias voltage for a symmetricdouble-floating probe is approximated by $\begin{matrix}{I_{DFP} \cong {I_{sat}^{+} \cdot {\tanh\left( \frac{e\left( {V_{0} - V_{DFP}} \right)}{T_{e}} \right)}}} & (1)\end{matrix}$where I⁺ _(sat) is the ion saturation current, V₀ is the floatingpotential of the probes when no bias is applied, V_(DPF) is thedifferentially applied probe voltage, and T_(e) is the apparent electrontemperature. Equation 1 may be modified to take into non-idealsituations that include effective probe area expansion with increasingbias potential, asymmetric probe areas, non-uniform plasmas andnon-Maxwellian electron energy distributions. The conventional analysisthat results in Eq. 1 is for a DC floating DFP case, but there isnowhere in the prior art where workers experimentally or theoreticallyexamined the DFP I-V characteristics when the DFP is placed on anRF-biased work piece. Without analysis or experimentation, one maysuspect that an RF-bias 136 would distort the I-V characteristics andpotentially confound the probe technique and analysis. Such distortioncould be due to RF modulation of the plasma sheath boundary andpotential disruption of electron currents 122 and 126 to the probeelectrodes 48. As a result the I-V characteristics properties underRF-biased conditions might not resemble 128 or follow Eq. 1 when thewafer 14 is placed on an RF biased electrode, as often done in plasmaprocessing systems.

To see how the DFP sensor responds in the presence of an RF-bias signal,an arrangement shown in FIG. 8 a was prepared in which several pairs ofsquare DFP aluminum thin film probes 48 (˜0.7 cm²) weresputter-deposited over a 200 mm silicon wafer 14 with a ˜1.5 um thickinsulating oxide 140. The probes were attached to an external ACfloating power source 142 that was swept with at peak differentialvoltage about 40 V_(p-p) at 30 Hz to simulate a slow voltage sweep.Currents were sensed with a floating resistor and a differential voltageprobe. The silicon wafer was placed in a commercial oxide etchingchamber which had a 13.56 MHz inductively-coupled plasma source withplanar induction coil (top) and a 13.56 MHz capacitively-coupledelectrode (bottom) in a manner commonly used in high-density plasmasemiconductor manufacture, i.e. with the wafer on the bottom ofcapacitively-coupled electrode 144 to provide an RF induced self-bias inorder to accelerate ions to the wafer surface. In order to inject the ACsignals without disrupting the RF induced self-bias, two pairs ofbalanced series RF resonant choke filters tuned to 13.56 MHz 146 and27.12 MHz 148 were placed in line with low pass filters 150 to allowexternal injection of the 30 Hz AC sweep yet allow the wafer and DFPs tofollow RF signal and obtain an effective self-bias of several hundredvolts of −100 to −300 V. Such RF blocking circuits have been commonlyused in plasma-based diagnostic systems and manufacturing fixtures inorder to mix DC or low frequency AC signals without perturbingconcurrent RF-signals and circuitry within the processing system. Themeasured I-V trace of the DFP under RF self-bias conditions is shown inFIG. 8 b. The oscilloscope trace shows the 30 Hz sinusoidal sweep 152and the responding DPF current 154. After accounting for smalldisplacement current offset due to parasitic capacitances between theprobes and the shape of the sinusoidal DFP voltage sweep, it is clearthat the measured I-V trend is virtually identical to that seen with noRF self-bias. This suggests that the DFP diagnostic method caneffectively be used to sample plasma charged-particle characteristicseven in the presence of an RF-induced negative self-bias of severalhundreds of volts.

To corroborate the unexpected result and to make certain that thebalanced RF blocking filter mechanism has no bearing on the experimentalresults, an analysis of the dual-floating Langmuir probe theory was madewhich included the effects of a high-amplitude common-mode RF signal, asinduced by the RF self-bias, on top of the floating V_(DPF) signal inthe electron currents to the probes. The analysis assumes that iontransport across the plasma sheath above the RF-biased wafer isrelatively constant with time as is the case when operating at wellabove 1 MHz. Provided this assumption, the classic I-V characteristicsof the DFP diagnostic method given in Eq. 1 are retained, despite thepresence of the high-amplitude common-mode RF signal. Thus bothexperimental and theoretical analysis show that the DFP diagnosticmethod can provide good measurements of ion saturation currents andapparent electron temperatures in accordance with the classical DFPdiagnostic method. It should be noted that the experimental result andtheoretical analysis for the DFP diagnostic as disposed on an RF-biasedwork piece have not been discussed or taught in the prior art, yet theunique result has great utility in that it provides a viable in situplasma sensor for the apparatus of the present invention.

In order to practically implement the DFP diagnostic technique on an insitu sensor module, it is necessary to provide a floating probe biasmean that may be completely contained within the electrically floatingapparatus. Such a means is illustrated in FIG. 9, wherein the DFP pads48 are capacitively-coupled through isolation capacitors 156 and 158 toan input signal from a storage capacitor 160 and charge-pump circuit 162tied to a floating common 164. In this circuit, two field effecttransistors (FETs) 166 and 168 are used to allow the flow of currentthrough DFP device and to reset the charging condition prior to samplingthe DFP current and voltage. A sampling resistor 170 is used to senseDFP current and a voltage divider 172 is used to sample the DFP biasvoltage level. The signal trends against time are also illustrated inFIG. 9. The representative signal levels show how the DPF current isallowed to flow through the circuit and plasma 34 once the DFP_(pulse)FET 166 is turned on. At this time the apparatus microprocessor samplesthe I_(DFP) current and DFP bias level. For repeated sampling under thesame or different DFP bias levels, it is necessary to equalize or resetthe charge state of the isolation capacitors 156 and 158. This isaccomplished through a shunting FET 168 that is turned on between DFPpulses and A/D sampling. The circuit illustrated in FIG. 9 allows one tosample the DFP current and voltage dynamically as the bias voltage andresponding DFP current relaxes in time or, alternatively, at discretelypulsed intervals and bias level as controlled through the charge-pumpcircuitry and timing of the DFP pulse and A/D sampling. Moreover, thecircuit of FIG. 9 can be adapted to other electrically floating sensingdevices such as photodiodes detection devices for light emissiondiagnostic methods, thermistors, various micro-electro-mechanical (MEM)sensors and other impedance-based sensing devices.

The topographically dependent charging (TDC) device is another componentthat has several possible applications in the apparatus of the presentinvention. FIG. 10 illustrates the general principles of the TDC device52. The TDC device is generally comprised of a substrate 174, a lowerconductive electrode 176, a patterned insulator with relatively highaspect ratio lines or holes with sub-micron dimensions 178 and a topconductive electrode 180. The typical dimensions for a TDC device is anopening of <1 um with aspect ratios that are about 5 or greater. Forexample, a typical TDC device may have a hole or line opening of 0.3 umand feature depth of 1-1.5 um. When exposed to a plasma, the differencesin ion and electron transport to the top electrode 180 and the buriedbottom electrode 176 results in a positive potential on the bottomelectrode. This charging effect is well known in the industry since suchcharging effects can influence the quality producing high aspect ratiosub-micron features and can lead to surface charging effects that resultin device damage during semiconductor IC manufacture.

This charging effect is well known in the industry since such chargingeffects can influence the quality producing high aspect ratio sub-micronfeatures and can lead to surface charging effects that result in devicedamage during semiconductor IC manufacture. Such surface charging andpotential charge damage effects are dependent upon the plasma conditionsand spatial uniformity of the plasma. Thus one use of the TDC in thepresent invention is a monitor of static charges across various TDCdevices with varying aspect ratios for spatial and temporal measure ofsurface charging effects.

An application of the TDC device is also shown in FIG. 10. In this casethe TDC 52 provides a source of DC power to various electricalcomponents and sub-systems that have been illustrated in the variousproposed architectures. Since the TDC devices provide a DC potentialfrom the charged carrier transport against the plasma boundary, they maybe used as a continuously charged power source when the plasma isactive. As with a battery or storage capacitor, a TDC-based power sourcewould also need a DC-DC converter to adjust and regulate the power toappropriate DC voltage levels. In this application the top electrode 180of the TDC is used as the local common. As an example, the TDC may powerone DC-DC converter 182 for the charge pump and sampling circuitry 184for the DFP diagnostic system 186 and a second DC-DC converter 188 topower a local microprocessor 190 used for sampling, A/D conversion andserial communications. The power available from the TDC device or bankof TDC devices is limited by the ion current flux collected at the baseelectrode 176 and effective DC potential when electrically connected toa load. A typical processing plasma system may have ion current fluxdensities on the order of 1-5 mA/cm², and under typical RF self-biasvoltages of a few hundred volts, the charging of a TDC device may beseveral 10s of volts. Thus the power density that may be derived is asmuch as 0.1 W/cm² for a TDC structure that has 40% open ion collectionarea. Thus a 1-2 cm² TDC device or bank exposed to a plasma could beintegrated with a DC-DC converter to provide the same DC power of a 3.2V coin battery with a peak trickle-current rating of about 30 mA. Ingeneral, the power derived from the TDC device may be used as anauxiliary source or DC power for sensor devices or for recharging of themain charge capacitor or battery.

While we illustrate a TDC device for providing auxiliary electronicpower from the plasma process, other chargeable structures, such as athin film capacitor stack, which is patterned on the wafer surface, mayalso serve this purpose. As with the TDC structure, such a device canaccumulate a net DC charge and thus provide a differential DC voltagewhen it is exposed an RF bias in a plasma process. This DC voltage maythen be regulated by a DC-DC converter in order to power the deviceelectronics or to recharge the device battery. The principle of such acharging capacitor 192 is illustrated in FIG. 11. In this illustration,the charging capacitor may be formed from a bottom conductive electrode194 attached to the base of the substrate or wafer 174, an insulatingthin film 196 and a top conductive electrode 198. Various multi-layer,inter-digitated capacitor configurations can be used to increase thecapacitance. When the substrate is exposed to a plasma and, moreparticularly, to an RF bias, a net DC voltage is sustained across theinsulator. With the appropriate selection of insulating film thickness,dielectric constant and capacitor area, a charging device may beconstructed to provide the appropriate DC voltage and current capacitylevels for auxiliary power generation. For example, such an electricallyfloating capacitive device with collection area of 10 cm² can provide anet DC voltage on the order of 1 to 10 V and draw currents on the orderof 100s of microamperes.

FIG. 12 a illustrates an application of the TDC for the apparatus of thepresent invention wherein the loaded I-V characteristics or “load-line”of the TDC device is probed. In this configuration, the bottomconductive electrode 176 of a TCD sensor device 52, or several TDCdevices of varying aspect ratios, are individually tied to a FET 200.The FET is operated in a linear resistive region through a controlledgate voltage signal 202. The output of the FET is connected to a currentsensing resistor 204 which is tied to common or the top electrode of theTDC 180 and, thereby, allows one to sample the draining TDC current 206as the loading resistance of FET is changed. A high impedance voltagedivider 208 is used to sample the voltage of the loaded TDC device. Inthis manner both one may collect a load-line characteristic of the TDCdevice when exposed to the processing plasma environment.

A typical load-line characteristic is illustrated in FIG. 12 b as itmight arise from a TDC structure. TDC structures with different aspectratio may provide somewhat different load lines when exposed todifferent plasma processing environments and RF biases. As describedearlier, the TDC device voltage and current arises from the differencesbetween net ion and electron transport through the TDC structure whenexposed to a plasma. When the top and bottom electrodes of the TDCdevice are connected (short circuit), it is possible to drive a currentthrough the connection as the net negative electron flux is allowed toneutralize the net positive ion flux that reaches the bottom electrode.When the connection between top and bottom electrodes is open (opencircuit), a voltage is sustained due to the imbalance ofcharged-particle fluxes and net accumulation of positive charge at thebase of the device. The exemplary load-line characteristic of FIG. 12 bshows the response of the TDC device's current and voltage as it isloaded between the shorted and open conditions. Aside from the aspectratio and scale of the TDC structure, the TDC device load-line isdetermined by factors that influence the spatial density, effectivemass, phase velocity and energy of charged species to the TDC. Thesefactors can include gas pressure, plasma chemistry, power density,chamber surface conditions and RF bias levels. As such, a detailedmeasure of the load line response of one or several TDC devices canprovide far more subtle information about the plasma processingconditions than just the open-circuit voltages or short-circuitcurrents. Moreover, the load-line provides a power line that can be usedto match the output of TDC devices to DC-DC converter circuitry whenusing the TDC devices as a DC power source in the apparatusarchitecture.

It should be noted that while a FET is specifically used in thisillustration as the means by which to collect the loaded I-Vcharacteristics of the TDC sensing device, there may be other methods bywhich to electrically load the TDC device and sense the I-V responseand, thereby, quantify the state of charged-particles of the plasmaadjacent to the sensor.

While any practical method of fabrication may be used to form theprobing component of the apparatus of the present invention, there areseveral pragmatic issues that have bearing on its ultimate use. Some ofthese issues include the selection of materials, limit in thermal rangeof operation, profile or height and balance, and chemical robustness andcompatibility with the processing vacuum environment, and deign featuresto limit wear of components after cycled use. The following listemphasizes some of the common design constraints.

-   -   1) The probing component must be vacuum compatible and must not        steadily outgas any significant compounds that would contaminate        the process or process chamber. Also, when placed under high        bias potential, the sputtered surface materials should not        contaminate the process or process chamber.    -   2) For most applications related to plasma processes, the        electrical components and materials placed into the vacuum        processing chamber should be able to operate at the peak        temperature levels usually observed. As an example, for common        etching operations materials that are rated to 125° C. are        appropriate.    -   3) For complex hybrid or ASIC-based circuitry that contain        multiple interconnections to other sub-system electronics,        hermetically sealed packaging should be used or monolithically        thick dielectric coatings should be considered with optional use        of metal electrostatic shielding from deposited thin films.    -   4) The overall height of the probing component that enters into        the vacuum processing chamber should be small enough to pass        through conventional load-lock gate valves and associated slits        with the aid of conventional handling systems and robotics. This        clearance height is usually 1 cm or less. The probing component        should also be well balanced for manipulation with common        handling mechanisms such as lifting pins, robotic paddles and        rollers.    -   5) The probing device must be electrically self-contained and        operable when placed in a plasma and stimulated with an RF-self        bias, as would be the case for wafer or work piece in a        conventional plasma processing system.    -   6) The apparatus should be constructed so as to allow wireless        communications in either or two modes: first, in real-time from        within processing system and second, post process download from        the processing system chamber or load lock or from outside the        processing system altogether.    -   7) The features and scale of sensor devices within the probing        apparatus should be relatively small to allow spatial resolution        of surface temperatures or plasma body properties. The scale of        each sensor is preferably, but not limited to, 1 cm².    -   8) In order to capture a transient response of a process, it is        desirable that the storage or reporting or real-time data be        triggered by a sensor measurement such as an anticipated signal        threshold, signal slope, or statistical deviation. As such it is        desirable that apparatus be able to record sensor responses just        before or at the advent of the process being monitored.

While thermal sensors, the DFP device and the TDC device have beenmentioned in detail here, it is clearly understood by one skilled in theart that the apparatus may include any number of additional sensors.These may include MEMs devices, optical sensor, bulk resistivity devicesthat are sensitive to rates of etching, curing or deposition orinducement or magnetic fields. In some processes, MEMs devices might beparticularly useful sensors in that they are often fashioned frommaterials that are compatible with plasma-based process environments.One examples of a useful MEMs device is a CMOS-based resonant beamsensor. Such sensors use a micro-machined cantilevered mechanism whosestimulated resonant frequency is dependent upon thermal and massproperties of the beam when exposed to the heat flux of the plasma,gaseous chemical absorbance, or mass changes due to reactive gas etchingor deposition.

Some examples of useful MEMs sensor technology include the followingdevices. A single-chip resonant beam gas sensor as described byHagleitiner et. al, “A single-chip CMOS Resonant Beam Gas Sensor” 2001IEEE International Solid-State Circuits Conference, Feb. 6, 2001. Thisdevice which was designed to detect the mass absorption of volatileorganic compounds could be used in conjunction with present invention tomonitor the mass absorption, accumulation or removal as related to aplasma assisted process. Another example is a Hall magnetic sensor asdescribed by Frounchi et al. “Integrated Hall Sensor Array Microsystem”2001 IEEE International Solid-State Circuits Conference, Feb. 6, 2001.This integrated micro-sensor is a device for monitoring magnetic fieldstrengths and could be used to monitor magnetic fields that areroutinely used to enhance in plasma processing system to either controlor enhance the process. Another device is micro-scale retarding fieldenergy analyzer (or ion energy analyzer) as described by Blain, et al.“High-resolution submicron retarding field analyzer for low-temperatureplasma analysis” Applied Physics Letters, Vol. 75, pp 3923, 1999. Thisdevice shows how a submicron-level ion energy analyzer could beconstructed as a sensor on a patterned wafer. Such a device could beeffectively operated with an electrical variant of dual floating probecircuitry as described earlier in order to obtain ion energydistributions for this type of analyzer. Yet another class of sensingtechnology are various integrated thin film optical photo sensor orphoto spectrometer sensors that incorporate thin-film bandwidth specificoptical filters that are fabricated with conventional CMOS chipfabrication methods. Optical emission and absorbance has been widelyused to study the ultraviolet, optical and infrared spectra ofprocessing plasmas for process development and control.

Optical sensors such as photodiodes, with or without passive opticalfiltering, can also be used to measure optical emission as radiated tothe surface of the work piece. Use of multiple optical emissions sensorscan enable the measure of multiple wavelength intensities as would berequired for in situ actinometry or other optical emission spectroscopymethods.

The self-contained DC power source or reservoir may be formed providedby a number of means. Low-profile, commercially available coin-stylebatteries are widely available and can be used if they meet thermalspecifications, trickle current levels, mA/hr ratings and are packaged(i.e. hermetically sealed) so as not to outgas electrolytic compounds.Also it is possible to use thin-film, multi-layer charge-capacitorsdevices which may be re-charged prior to use or within the plasmaprocess from an optional TDC device and DC-DC converter. Yet othervariations of low profile batteries, re-chargeable batteries, andcharge-storage capacitors can be incorporated into the design to providepower to the apparatus during the plasma process or for externaltesting, device configuration and calibration when outside of the plasmaprocessing system.

Although there is illustrated and described specific structure anddetails of operation, it is clearly understood that the same were merelyfor purposes of illustration and that changes and modifications may bereadily made therein by those skilled in the art without departing fromthe spirit and the scope of this invention.

1-10. (canceled)
 11. A method for obtaining measurements in a plasmaprocessing system comprising the steps of: a) disposing a measurementprobe into a plasma processing system, the measurement probe comprisinga substrate and at least one electrically floating sensor disposed onthe substrate; b) obtaining measurements of a plasma property in theplasma processing system using the measurement probe; c) processing andstoring measurement data on the measurement probe; and d) wirelesslymeasurement data from the substrate outside the plasma processingsystem.
 12. The method of claim 11 further comprising the step ofutilizing the plasma to power a self contained power source integratedinto the substrate.
 13. The method of claim 11 wherein the measurementsare non-invasive to the plasma process.
 14. The method of claim 11wherein the measurement data are wirelessly transmitted in real timefrom the substrate outside the plasma processing system.
 15. The methodof claim 11 wherein the electrically floating sensor comprises a dualfloating Langmuir probe.
 16. The method of claim 11 wherein theelectrically floating sensor comprises a topographically dependentcharging device.
 17. The method of claim 11 wherein the electricallyfloating sensor comprises a photodiode.
 18. The method of claim 11wherein the substrate is a silicon wafer.
 19. The method of claim 11,further comprising the step of monitoring process conditions in theplasma processing system using the measurement data.
 20. The method ofclaim 11, further comprising the step of using the measurement data todetermine rates of surface modification occurring in a plasma process.21. The method of claim 11, further comprising the step of using themeasurement data to determine the quality of surface modificationoccurring in a plasma process.
 22. The method of claim 11, furthercomprising the step of using the measurement data to determine an effecton process results due to variations in at least one of: a) gas pressurewithin the plasma processing system; b) plasma chemistry within theplasma processing system; c) power density within the plasma processingsystem; d) chamber surface conditions within the plasma processingsystem; and e) RF bias levels within the plasma processing system.